1. Field of the Invention
The present invention is related to a super luminescent diode that emits light having a central wavelength within a range of 0.95 μm to 1.2 μm. Particularly, the present invention is related to a super luminescent diode having a window region layer, and a manufacturing method thereof.
2. Description of the Related Art
Super luminescent diodes are semiconductor light emitting elements which are capable of emitting low coherence light having a wide band spectral shape, similar to light emitting diodes. Super luminescent diodes are also capable of emitting light having high directionality, up to several tens of mW, similar to semiconductor lasers. The basic structure of super luminescent diodes is similar to that of semiconductor lasers, except that a configuration that suppresses laser oscillation is adopted.
In order to suppress laser oscillation, it is necessary to suppress the reflectance at facets of super luminescent diodes. Known examples of means for suppressing reflectance include: (1) Anti Reflective (AR) films; (2) oblique optical waveguide path structures; (3) non-excitable facet structures; and (4) facet window structures. AR films are easy to generate, but have the drawbacks that laser oscillation occurs regardless of their provision, and that it is difficult to obtain high output when they are provided. The oblique optical waveguide path structures suppress oscillation by slightly inclining optical waveguide paths from optical waveguide facets. The oblique optical waveguide path structures are easy to generate, and is highly effective in suppressing laser oscillation. However, beam cross sections become distorted, and coupling efficiencies with optical fibers and the like deteriorate, because the emitting direction of light beams is inclined. The non-excitable facet structures suppress oscillation, by providing non-excitable layers that absorb light and suppress oscillation along optical waveguide paths. Although the non-excitable facet structures are easy to generate, there is a problem that heat is generated de to the light absorption, as the result of which element life is shortened.
The facet window structures suppress oscillation by providing window regions on facets which are not the light emitting facets optical waveguide paths by embedded growth, and diffusing light. Because embedded growth is necessary to form the window regions, difficulties in generation are increased. However, element properties are most favorable with the facet window structure. Note that it is necessary for the portion that corresponds to the window region to be embedded with a material that has a greater energy gap and a lower refractive index than the material than the material of an active layer. For example, the super luminescent diode having a GaAs substrate disclosed in Japanese Unexamined Patent Publication No. 5(1993)-243608 employs AlxGa1-xAs as the material of an active layer, and AlyGa1-yAs (x<y) as the material of the window region.
Recently, research of medical diagnosis and the like utilizing light is being advanced. It is known that low coherence light having a wide full width at half maximum spectrum is effective in optical diagnosis of living tissue. Particularly, low coherence light having a central wavelength within a range of 0.95 μm to 1.2 μm is not likely to be influenced by absorption by water, which is the main component of living tissue, and therefore is preferred as a low coherence light for optical diagnosis. Titanium sapphire lasers and the like are conventionally utilized as light sources that emit this type of low coherence light. However, many of these lasers are costly and difficult to handle.
Meanwhile, a super luminescent diode that emits low coherence light having a central wavelength within a range of 0.95 μm to 1.2 μm is known. However, this super luminescent diode is of the oblique optical waveguide path structure, and has a distorted beam cross section. Accordingly, this super luminescent diode is not suitable as a light source for optical diagnosis.
The present inventor focused on a super luminescent diode having a window structure at a facet of an optical waveguide path, as a super luminescent diode which is capable of emitting light having an undistorted beam cross section. However, a problem was discovered in that desirable element life is not obtainable with a super luminescent diode having a conventionally known structure.